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Spicule formation in the calcareous sponge Sycon ciliatum
Authors:Philip W Ledger  Dr W Clifford Jones
Institution:(1) Department of Zoology, University College of North Wales, LL57 2UW Bangor, Gwynedd, UK;(2) Department of Zoology, U.C.N.W., LL57 2UW Bangor, Gwynedd, UK;(3) Present address: Laboratoire d'Histologie et Biologie Tissulaire, Université Claude-Bernard, 43 boulevard du 11 Novembre 1918, 69621 Villeurbanne, France
Abstract:Summary The spicule primordium is formed in an intercellular cavity within a group of sclerocytes. This cavity contains organic material which ensheaths the growing spicule but does not appear to determine the nature of the mineral morph (magnesian calcite) or the crystallographic orientation of the spicule. The tip of each growing spicule ray is seated in a lsquodense cuprsquo in the cytoplasm of the sclerocyte concerned. Both ends of monaxons are initially inserted each into a dense cup. As rays elongate the sclerocyte membrane around the tip becomes invaginated and forms a system of lsquoconverging spacesrsquo that possibly indicate high secretory activity in that region. Spicule growth involves the displacement and expansion of the organic sheath by the enlarging spicule. Fully formed spicules which are exposed to the mesohyl become surrounded by collagen fibrils. However, these fibrils are in no way concerned with the process of mineral deposition and are never found within the spicule calcite.
Keywords:Spicules  Sponge  Sycon ciliatum  Calcite  Biomineralization
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